Projection video display (PVD) systems based on digital mirror devices (DMDs) have become quite popular owing, in part, to their accurate color rendition, resolution, display size and cost. However, DMD-based rear-projection PVD systems require an elongated optical path between DMD and screen to magnify their images from a fraction of an inch to several feet wide. Front-projection DMD-based rear-projection PVD systems have at their disposal the distance separating the projector from the rear-projection screen. Rear-projection DMD-based rear-projection PVD systems rely in part on cabinet depth to provide that distance: the larger the rear-projection screen size, the greater the required distance and cabinet depth.
Liquid crystal displays (LCDs) and plasma displays, which compete with DMD-based rear-projection PVD systems, do not magnify images and consequently tend to be relatively shallow, perhaps on the order of a few to several inches. Further, their depth is fairly independent of display size. Consumers prefer a shallower cabinet, so reducing cabinet depth in DMD-based rear-projection PVD systems to maintain their competitiveness is an ongoing challenge.
One way to reduce cabinet depth is to use mirrors to fold the optical path. However, once the image is incident on the projection screen, further optical processing is necessary to collimate the light so it is steered toward the viewer. Moreover, that collimation should preserve the intensity and fidelity of the image; it should not attenuate or distort the image. Unfortunately, ever-thinner cabinets cause the angle of incidence to climb. The next generation of DMD-based rear-projection PVD systems may have angles of incidence around 75° relative to normal (90° being parallel to the rear-projection screen). This high angle of incidence presents a significant collimation challenge, and one that the prior art is unable to address without unacceptably dimming or distorting the image or being prohibitively expensive and therefore impractical for use in commercial DMD-based rear-projection PVD systems.
What is needed in the art is a rear-projection screen that can collimate light incident at a large angle relative to normal. What is further needed in the art is a method of rear-projection collimation that can accommodate high angles of incidence. What is still further needed in the art is a DMD-based rear-projection PVD systems that has a lower cabinet depth to screen-size ratio than prior PVD systems.